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This butterfly has extreme color vision

Butterflies may not have a human’s sharp vision, but their eyes beat us in other ways. Their visual fields are larger, they’re better at perceiving fast-moving objects, and they can distinguish ultraviolet and polarized light. Now, it turns out that one species of swallowtail butterfly from Australasia, the common bluebottle (Graphium sarpedon,pictured), known for its conspicuous blue-green markings, is even better equipped for such visual tasks. Each of their eyes, scientists report in Frontiers in Ecology and Evolution, contains at least 15 different types of photoreceptors, the light-detecting cells required for color vision. These are comparable to the rods and cones found in our eyes. To understand how the spectrally complex retinas of butterflies evolved, the researchers used physiological, anatomical, and molecular experiments to examine the eyes of 200 male bluebottles collected in Japan. (Only males were used because the scientists failed to catch a sufficient number of females.) They found that different colors stimulate each class of receptor. For instance, UV light stimulates one, while slightly different blue lights set off three others; and green lights trigger four more. Most insect species have only three classes of photoreceptors. Even humans have only three cones, yet we still see millions of colors. Butterflies need only four receptor classes for color vision, including spectra in the UV region. So why did this species evolve 11 more? The scientists suspect that some of the receptors must be tuned to perceive specific things of great ecological importance to these iridescent butterflies—such as sex. For instance, with eyes alert to the slightest variation in the blue-green spectrum, male bluebottles can spot and chase their rivals, even when they’re flying against a blue sky.